Method and apparatus for compensating distortion on long loaded lines



Nov. 8, 192 7.

H. NYQUIST METHD AND APPARATUS FOR COMPENSATING DISTORTION ON LONGLOADED LINES Filed NOV. 2. 1925 /NVERTER Zyl* ` IN VEN TOR MTORNEYPatented Nev. s, 1,921.

'unirse STATES P'rrNr Fries.

HARRY NYQUIST, JACKSON HEIGHTS, NEW YORK, .ASSIGNOB '.-lO AMERICAN TELE#:RHONE AND TELEGRAPH COMPANY, A CORPORATION F NEW YORK.

' miren AND ArrAnA'rUs non. COMPENSATING nIsroBrIoN oN Lone LOADEDLINES.

Applicatiomled November 2, 1923. Serial No. 672,945.

n Vobje/c/t of my invention'is to provide apparatus and a method tocompensate for the distortion on a long loaded line due t0 thedifferential retardation of the different frequencies of the voicerange. Another object of my invention isl to provide for compensatin thedifferential retardation of a range -o frequencies to be transmitted incase the transmission line has the property of retarding thosefrequencies unequally.

Another object of mylinvention is to provide practicable means forinverting a range of frequencies. These and other objects of myinvention will be made apparent in the 16 following specification andclaims taken with the accompanying drawings, in which I have illustratedan example .of the invention which I now proceed to describe. lt will beunderstood that the followin specication relates more particularly to tiis embodiment of the invention and that its scope will be dened in theappended claims.

Referring to the drawings, Figure 1 is a diagram showing a four-Wiretransmission line adapted for operation according to the principles ofmy invention; Fig. 2 is a diagram of a frequency inverter and Fig. 3'isa diagram of an attenuation equalizer.

In this example which I proceed to disclose, the voice frequency rangefrom zero up to nearly 2400 cycles per second is involved. To invert thefrequencies of this range means tosubstitute for any frequency n afrequency of 2400fn. Thus, a frequency of 900 cycles per second in thenormal rango' becomes 1,500 cycles per second in the inverted range.Devices to accomplish this function are indicated symbolically at 20,21, 20 and 21 in Fig. 1. These are shown 4o in detail in Fig. 2, whichwill be described later, but for the present it will suffice tounderstand that for each frequency component 'n going intoV such adevice as 20, there will come ont of it a frequency The usual two-wireterminals of the fourwire line of Fig. 1 are shown at 11l and 11. Theinput circuit 11 is balanced by the artiicial line N across thethree-Winding transformer 12, and the input from the line 11 goes bythecircuit 13 and repeating coils 14 to the amplifier or one-Wayt repeater15. The loaded line 16 comprises the 'spaced loading Acoils 17 and atsuitable intervals attenuation equalizers 18 and repeaters 19 arelocated. In this example, with an assumed length Z for the line, theattenuation equalizers 18 and repeaters 19 are placed at intervals ofZ/12. i

At an interval of Z/ei. there is the usual attenuation equalizer andrepeater combination 18, 19 with an interposedinverter 20. Then, for alength Z/2 the loaded line 16 contmues with attenuation equalizers andrepeaters at lntervals of Z/12 until at 3Z/4 another lnverter 21 isplaced, and then the repeatered loaded line continues for the re` mamingZ/4 in normal manner to the end at the two-wire terminals 11. madethrough the repeating coil 22 and conductors 23'to the balanced ointsbetween the line 11 and its network gl. v

Thus, it will be seen that for transmission from 11 to 11, thefrequencyrange is normal for the first fourth of the way, inverted forthe next half of the way and again normal f or the remaining fourth ofthe way.

Pruned numerals have been employed for the transmission from east toWest, which corresponds to that from west to east and n eed not bedescribed here in detail.

Tn a medium heavy loaded line the lower telephone frequenciesl will bepropagated W1th a speed of about 10,000 miles per second, whilefrequencies of 2,000 cycles per second wlll be transmitted with a speedof about 9,000 miles per second. On a line 1000 miles long thisdifferential speed will produce serious distortion 'at the receivingend. By inverting the frequencies over Connection is half the length ofthe line, such distortion will be effectively compensated.

The frequency inverter indicated symboli- I cally at20, v21, 20 or 21 isshown in the diagram of Fig. 2. Two audions are provided and thecircuits of their respective grids 33 and 33 comprise the respectivesecondary windings 32 and 32. The corresponding primary winding 31 isenergized by a source 30 of alternating current of 2400 cycles persecond. The input of voice frequency range coming over the line 16 tothe potentiometer 41 goes through the primary winding 40 associated withthe respective secondary windings 39 and 39 in the grid circuits. Thesegrid circuits are connected to the filaments 35 and 35 throughtentiometer 38 is provided for adjusting the balance between the twotubes. Filaments 35 and 35 .are glowed in series by current from thebattery 36 through the rheostat 37.

The audion plates 42 and 42 are in parallel plate circuits on thebattery 47 connected through the inductance 46. These plate circuitscomprise the respective primary windings 44 and 44 with the associatedsecondares 48 and 48. 49, 50 is a shunt, resonant at 2400 cycles, and 51is a lowass filter which cuts off transmission at a out 2100 cycles. Asuitable condenser 43 is inter osed as shown across the plate circuits.

the audion it is Well understood that the effect on the plate circuitsis due to the combined grid otential and plate potential.

Let the factor y which the plate potentialv must be divided to make itcoordinate with the grid potential be designated by u, and let -Arepresent this plate potential divided by p minus the grid potential,which is negative. Let E stand for the Voltage of the 2400 c cle currentimpressed on the grid. Let e ge the voltage due to the input currentimpressed on the grid.

Then the current in the plate circuit of one audion may be represented-with suiii-v cient accuracy for the present purpose by the expressionalso the current in the plate circuit of the other audion may berepresented by Accordin ly the resultant combined output will be t edifference which is 4AE+4E6. The first term, 4AE, represents themodulating frequency in the output and the second term, 4Ee, representsthe two sideA bands of fre uency respectively higher and lower than t emodulating frequency of 2400 cycles. This modulating frequency of 2400cycles represented. by the term 4AE, is remove that is, shunted out, bythe resonant branch 49, '50. The upper side band of frequency rangeabove 2400 cycles is removed or shunted out by the low-pass filter 51and there remains to be transmitted to the line 16 only the invertedvoice frequency range below 2400 cycles.

The foregoing discussion has been based on the assumption that the twoaudions have identical amplification constants and identical internal imedance. If these conditions are realized on y approximately it willstill be possible to et a very good balance, provided the impe ance ofthe output condenser detail in Fig. 3. This is a well-known piece ofapparatus and will be described only briey. The transformer 60, 61 .hasa d1- vided secondary in which the resistance 62 is interposed. Thisresistance 62 is shunted by a resonant combination 63, 64. .For theordinary voice range the resonance may appropriately be at about 2800cycles. Thus, it will be seen that for high frequencies in the voice rane the resistance 62 will be shunted by a ow impedance path and will notbe attenuated by the resistance 62, whereas, low frequencies in thevoice range will be more attenuated by the resistance 62 and in this waythe intensity over the voice range is substantially equalized. Theoutput from the secondary 61 goes through the potentiometer 65 to therepeating coil tial retardation of the frequency components of a rangeof frequencies to be transmitted and means interposed in said line toinvert said frequency range and means'also interposed again to invertthe frequency range and restore it to normal after transmission of theinverted range over substantially half the length of the line.

3. A long loaded line, repeaters therein,

a frequency inverter at a distance of about one-fourth the line lengthfrom the sending end and another frequency inverter at a distance ofabout three-fourths the line length from the sending end whereby thedifferent frequencies at the receiving end are in substantially the samephase relation as at the sending end.

4. A long loaded four-wire transmission line, one-way repeaters in eachcircuit-pair of conductors and two frequency inverters in eachcircuit-pair at a distance apart equal to about half the length of theline wherebytransmission each way is with the normal frequency rangeabout half-way and the inverted frequency range about half-Way anddistortion due to differential retardation is compensated.

5. The method vof compensating differential retardation for differentfrequencies of a range in a long loaded line which consists in invertingthe frequency range over about half the length of the llne.

6. The method of compensating the differential retardation fordifferentfrequencies on a long loaded line which consists in transmitting sfraction of the way with the mainder of the way with the normalfrenormal frequenc range, then inverting the quency range. frequenciesof t is range and transmitting -In testimony whereof, I have signed my10 about half the length of the line with the name to this specificationthis 1st day of N0- 5 inverted frequency range, then again invertvember,1923.

ing the frequenc' range and thereby restoring it to norma andtransmitting the re- HARRY NYQUIST.

